The present invention relates to a boiling water nuclear reactor, and particularly relates to a start-up process of a natural circulation nuclear reactor in which circulation flow rate is secured by the hydrostatic head difference between the outside and inside of a reactor core (hereinafter also simply referred to as "core").
In a current water nuclear reactor, cooling water is circulated into a core by recirculation pumps at the start-up time after ordinary shut-down of the nuclear reactor (hereinafter also simply referred to as "reactor"), and the cooling water is heated by nuclear reaction by withdrawing control rods to thereby make the cooling water high in temperature as well as pressure. At this time, since the core is being cooled by forced circulation, the cooling water is in a state of single-phase flow. Being heated, the cooling water is made to transit monotonously from the state of single-phase flow into a state of two-phase flow to make it possible to perform stable starting-up of the reactor.
On the other hand, there is a boiling water reactor of the type in which at the start-up time, at least in a period from a non-critical state to a state in which an isolation valve is opened so that steam is discharged from the reactor, a core is cooled by natural circulation. For example, in a natural circulation reactor provided with no recirculation pumps, a hydrostatic head difference between the outside and inside of a shroud enclosing a core is used as driving force for the natural circulation of the cooling water in the core. Accordingly, if the cooling water is heated in the core by nuclear reaction at the start-up time after ordinary shut-down of the reactor, the cooling water outside and inside the shroud is circulated at a low flow rate by driving force generated by a density difference due to a temperature difference. When the water temperature becomes high and the subcool temperature at an inlet of the core becomes lower than the maximum subcool temperature to start boiling which is determined by physical properties and a circulation flow velocity, boiling is generated in the core. At this time, the hydrostatic head difference between the outside and inside of the shroud increases because of generation of steam bubbles to thereby increase the circulation flow velocity. By this, the quantity of cooling of the core increases so that the cooling water in the core comes back into the state of single-phase flow. This operation is repeated so that the state of single-phase flow and the state of two-phase flow are alternated to thereby generate flow fluctuations. This unstable phenomenon becomes remarkable under low temperature where the vapor-liquid density ratio is large and continues until the subcool temperature at the core inlet becomes lower than the minimum subcool temperature to cause unstable phenomena.
In such an unstable phenomenon at low temperature two-phase flow, the degree of void reaction of nuclear fuel fluctuates because of occurrence of flow fluctuations so that there arises a problem that the stability of the core can not be improved.
Further, in order to avoid such an unstable phenomenon at low temperature two-phase flow by making the temperature of the cooling water rise up in the state of single-phase flow to a temperature as high as possible while delaying the start of boiling, it is necessary to heat the cooling water with an extremely low quantity of heating by nuclear reaction for a long time. In this process, however, the circulation velocity in the core is so low that a phenomenon of thermal stratification occurs in the cooling water in a lower plenum in a pressure vessel and low temperature water stays in the lower plenum. Accordingly, when most of the cooling water becomes high temperature to start boiling, the low temperature water in the lower plenum flows into the core because of increase of the core circulation velocity to thereby generate a similar unstable phenomenon. Further, since the cooling water is heated with an extremely low quantity of heating by nuclear reaction, it takes a very long time for the start-up of the reactor to thereby extremely lower the economy in connection with the running of the reactor.
In a conventional system for preventing such an unstable phenomenon at low temperature two-phase flow, as disclosed in JP, A, 59-143997 and JP A 59-217188, at the start-up time of a natural circulation reactor, heat is supplied from a house boiler used in service inspection to cooling water in a pressure vessel of the reactor to raise the temperature of the cooling water and thereafter heating by nuclear reaction is started to thereby prevent lowering of core stability due to flow instability in the low temperature two-phase flow. In an alternative conventional process, as disclosed in JP, A, 60-69598, the temperature of a coolant inside a pressure vessel is raised through a heat exchanger so that the subcool temperature at a core inlet is set within a range smaller than the minimum sub-cool temperature to cause unstable phenomena, and thereafter output increase is started to thereby secure the stability of the core at the start-up time of the reactor.
In each of the above conventional techniques, equipment of heat is supplied by equipment inside/outside a housing and no improvement is made on the equipment of the reactor primary cooling system, the start-up process and the start-up characteristics. Further, heat of nuclear reaction is not used to raise the temperature of the cooling water, so that not only heat loss is generated for heat generation by a boiler and transportation of the heat but also in order to obtain the same quantity of heat as that of nuclear reaction, it is necessary to provide a large-scale boiler or it takes a long time for the start-up of the reactor, resulting in reduction in economy. Further, since a heat exchanger and a heat supply system are provided outside/inside a housing or inside a pressure vessel to increase the temperature of the cooling water, pipings and a control system are required to make the structure of the reactor complicated and there arises a problem that the economy and reliability can not be improved.
Further, even in the case of employing a process in which cooling water is heated with an extremely low quantity of heat of nuclear reaction for a very long time to thereby avoid the unstable phenomena in low temperature two-phase flow, there arises a problem that it takes a very long time for the start-up of the reactor and the economy in connection with the start-up of the reactor can not be improved.